The present invention relates to compound semiconductor devices, and more particularly relates to high-speed operating heterojunction bipolar transistors.
In heterojunction bipolar transistors (which will be herein referred to as xe2x80x9cHBTsxe2x80x9d), emitter injection efficiency (i.e., the ratio of electric current injected into a base layer to the entire emitter current) is increased by using, for an emitter layer, a semiconductor material having a wider bandgap than that of a material for a base layer. In other words, a heterojunction formed between the emitter layer and the base layer is utilized to increase the emitter injection efficiency. Thus, the thickness of the base can be reduced and thereby the impurity concentration in the base layer can be increased. Therefore, HBTs can operate at a high speed.
Moreover, HBTs utilizing the heterojunction between a base layer and a collector layer as well as the heterojunction between an emitter layer and the base layer are called xe2x80x9cdouble heterojunction bipolar transistorsxe2x80x9d (which will be herein referred to as xe2x80x9cDHBTsxe2x80x9d). In DHBTs, compared to HBTs including a homojunction interface between a base layer and a collector layer, collector breakdown voltage is increased while collector-emitter offset voltage is reduced. Thus, operation properties of DHBTs can be advantageously improved with a low voltage applied.
In DHBTs, however, a semiconductor material having a wider bandgap than that of a material for a base layer is used as a material for a collector layer, and therefore an energy barrier is formed in the conduction band of the heterojunction interface between a collector layer and the base layer. The energy barrier blocks electrons"" movement, resulting in the electron blocking effect of lowering current gain and degrading transistor properties. Then, as a conventional technique for preventing the electron blocking effect, a method in which the height of an energy barrier formed at a heterojunction interface is reduced by providing a setback layer interposed between a base layer and a collector layer is well known.
Note that a heterojunction interface means herein the interface between a semiconductor layer having a relatively narrow bandgap and a semiconductor layer having a relatively wide bandgap. Therefore, in a DHBT in which no setback layer is provided, the interface between a base layer and a collector layer is a heterojunction interface. On the other hand, in a DHBT in which a setback layer, e.g., formed of the same material as that of the base layer, is provided, the interface between the setback layer and a collector layer is a heterojunction interface.
FIG. 3 is a cross-sectional view of a conventional compound semiconductor device. More specifically, FIG. 3 is a cross-sectional view of a DHBT including a setback layer.
As shown in FIG. 3, a buffer layer 11 formed of a GaAs layer, a sub-collector layer 12 formed of an n-type GaAs layer, a collector layer 13 formed of an n-type InGaP layer, a setback layer 14 formed of an undoped GaAs layer, a base layer 15 formed of a p-type GaAs layer, an emitter layer 16 formed of an n-type InGaP layer, a contact layer 17 formed of an n-type GaAs layer, and an emitter cap layer 18 formed of an n-type InGaAs layer are stacked in this order on a semiconductor substrate 10 formed of a GaAs substrate which is a semi-insulator. In this case, GaAs having a narrower bandgap than that of a material for the collector layer 13 (InGaP) is used as a material for the setback layer 14. That is to say, the setback layer 14 is formed of the same material as that of the base layer 15. Moreover, a collector electrode 19 is formed on a region of the sub-collector layer 12 on which the collector layer 13 does not exist. A base electrode 20 is formed on a region of the base layer 15 on which the emitter layer 16 does not exist. And an emitter electrode 21 is formed on the emitter cap layer 18.
FIG. 4 is a diagram illustrating energy bandgaps in the FHBT shown in FIG. 3.
As shown in FIG. 4, each of the energy bandgaps of the emitter layer 16 and the collector layer 13 is wider than that of the base layer 15. It can be also seen from FIG. 4 that with the setback layer 14 provided between the base layer 15 and the collector layer 13, the height of the energy barrier formed at the heterojunction junction is reduced, and thus electrons can easily move from the base layer 15 to the collector layer 13.
However, the DHBT of FIG. 3, i.e., a conventional compound semiconductor device, still has a problem in which, although the height of the energy barrier seen from electrons can be reduced to a certain level by providing the setback layer 14, the electron blocking effect can not be completely prevented.
In view of the above-described problems, an object of the present invention is to ensure prevention of the electron blocking effect in a double heterojunction bipolar transistor.
To attain the above-described object, a first compound semiconductor device according to the present invention is assumed to include: an emitter layer; a base layer which is in contact with the emitter layer and formed of a first compound semiconductor; and a collector layer which is in contact with the base layer and formed of a second compound semiconductor having a wider bandgap than that of the first compound semiconductor. In the device, a delta doped layer having a higher concentration of an impurity than that of the collector layer is formed in a region of the collector layer located at about 10 nm or less from the heterojunction interface with the base layer.
In the first compound semiconductor device, the delta doped layer is provided in a region of the collector layer located in the vicinity of the heterojunction interface with the base layer. Thus, the thickness of an energy barrier formed at the heterojunction interface is reduced and electrons can easily pass through the energy barrier. Therefore, it is possible to suppress the energy barrier""s blocking of movement of electrons injected from the base layer to the collector layer, and thus the electron blocking effect can be reliably prevented. Accordingly, current gain is increased and thus transistor properties can be improved, resulting in, e.g., a highly effective double heterojunction bipolar transistor suitable for high-speed operation.
To attain the above-described object, a second compound semiconductor device according to the present invention is assumed to include: an emitter layer; a base layer which is in contact with the emitter layer and formed of a first compound semiconductor; a collector layer which is in contact with the base layer and formed of a second compound semiconductor having a wider bandgap than that of the first compound semiconductor. In the device, a delta doped layer having a higher concentration of an impurity than that of the collector layer is formed at the heterojunction interface between the collector layer and the base layer.
In the second compound semiconductor, the delta doped layer is provided at the heterojunction interface between the collector layer and the base layer. Thus, the height of an energy barrier formed at the heterojunction interface is reduced and electrons can easily go over the energy barrier. Therefore, it is possible to suppress the energy barrier""s blocking of movement of electrons injected from the base layer into the collector layer, and thus the electron blocking effect can be reliably prevented. Accordingly, current gain is increased and thus transistor properties can be improved, resulting in, e.g., a highly effective double heterojunction bipolar transistor suitable for high-speed operation.
A third compound semiconductor device according to the present invention is assumed to include: an emitter layer; a base layer which is in contact with the emitter layer and formed of a first compound semiconductor; and a collector layer which is in contact with the base layer and formed of a second compound semiconductor having a wider bandgap than that of the first compound semiconductor. The device further includes a setback layer provided between the collector layer and the base layer and formed of a third compound semiconductor having a narrower bandgap than that of the second compound semiconductor. And in the device, a delta doped layer having a higher concentration of an impurity than that of the collector layer is formed in a region of the collector layer located at about 10 nm or less from the heterojunction interface with the setback layer.
In the third compound semiconductor device, the setback layer is provided between the collector layer and the base layer and the delta doped layer is provided in a region of the collector layer located in the vicinity of the heterojunction interface with the setback layer. Thus, the height and thickness of an energy barrier formed at the heterojunction interface are reduced and electrons can easily go over or through the energy barrier. Therefore, it is possible to suppress the energy barrier""s blocking of the movement of electrons injected from the base layer into the collect layer, and thus the electron blocking effect can be reliably prevented. Accordingly, current gain is increased and thus transistor properties can be improved, resulting in, e.g., a highly effective double heterojunction bipolar transistor suitable for high-speed operation.
A fourth compound according to the present invention is assumed to includes: an emitter layer; a base layer which is in contact with the emitter layer and formed of a first compound semiconductor; and a collector layer which is in contact with the base layer and formed of a second compound semiconductor having a wider bandgap than that of the first compound semiconductor. The device further includes a setback layer provided between the collector layer and the base layer and formed of a third compound semiconductor having a narrower bandgap than that of the second compound semiconductor. And in the device, a delta doped layer having a higher concentration of an impurity than that of the collector layer is formed at the heterojunction interface between the collector layer and the setback layer.
In the fourth compound semiconductor device, the setback layer is provided between the collector layer and the base layer and the delta doped layer is provided at the heterojunction interface between the collector layer and the setback layer. Thus, compared to the case in which only the setback layer is provided, the height of an energy barrier formed at the heterojunction interface is further reduced and electrons can easily go over the energy barrier. Therefore, it is possible to suppress the energy barrier""s blocking of movement of electrons injected from the base layer into the collect layer, and thus the electron blocking effect can be reliably prevented. Accordingly, current gain is increased and thus transistor properties can be improved, resulting in, e.g., a highly effective double heterojunction bipolar transistor suitable for high-speed operation.
In the first through fourth compound semiconductor devices, the collector layer preferably includes a semiconductor layer formed of InGaP, InP or GaAs.
Thus, a double heterojunction bipolar transistor can be reliably achieved.
A method for fabricating a compound semiconductor device according to the present invention, which is assumed to be a method for fabricating either one of the first through fourth compound semiconductor devices, includes the step of forming the delta doped layer, while epitaxially growing a semiconductor layer that is to be the collector layer, by stopping the epitaxial growth and then introducing into the semiconductor layer an amount of impurity at least required to form a single atomic layer.
With the method for fabricating a compound semiconductor device according to the present invention, the capacitance between the base layer and the collector layer can be reduced, compared to the case in which the electron blocking effect is prevented by continuously changing the impurity concentration of the collector layer. Moreover, the amount of an impurity to be added as a delta doped layer into the collector layer can be accurately controlled with high reproducibility and without depending on epitaxial growth conditions, thus achieving stable transistor properties.